Prior art twisted wire pair [Ref. 2], employed in “balanced” or differential signaling addresses concerns of electromagnetic coupling such as crosstalk and electromagnetic interference (EMI) through wire pair design and shielding. Wire pair twist in particular, characterized by the “lay length” (length for one complete twist of the wire pair) of the pair, is helpful in ensuring that external noise coupled is, to the first order, the same in the two wires of the pair. Due to this property, “enhanced” cable categories employ very short lay lengths or tight twist, which also helps ensure that wires of the pair do not separate under mechanical stress induced, for example, by bending. Nevertheless, as discussed in U.S. patent application Ser. No. 11/654,168 and U.S. Pat. No. 7,449,639, wire pair twist results in other characteristics such as intra-pair skew, inter-pair skew, and mode conversion (differential signal to common-mode) along the length of the wire pair, which prove detrimental in high speed multimedia information transfer applications.
Mode conversion that results from intra-pair skew or individual wire impedance variations along the length of a twisted wire pair is particularly detrimental. The duration of differential signal transformed to common-mode leads to electromagnetic emissions from the wire pair, which may well couple into neighboring wire pairs carrying similar signals. Prior art therefore employs wire pair shielding, or a conductive cover around a wire pair that attenuates any electromagnetic radiation encountered. This shield typically takes the form of a conductive foil wrap around the twisted wire pair, and is reasonably effective (varying with radiating signal frequency) in absorbing wire-pair generated or external radiation. In order to ensure effectiveness of the shield, an uninsulated drain wire accompanies the twisted wire pair inside of the shield, making conductive contact with the shield. This drain wire helps ensure that the shield provides an effective, low-impedance return path for any common-mode or other stray signal generated from the twisted wire pair, thus containing the radiation from the twisted wire pair. Also, the shield responds to external radiation impinging upon the twisted wire pair, generating an opposing current that minimizes field transmission and signal coupling into the wires in the pair.
Nevertheless, shielding as implemented in twisted wire pair assemblies creates its own problems along the length of a cable. Shield foil wrapped around a twisted wire pair increases the capacitance of wires in the pair significantly, because each wire now has capacitance to the other and to the shield, therefore nearly doubling its capacitance. Because wire pair twist is done before foil is wrapped to form the shield, foil wrapped around the wire pair cannot be uniformly and equally wrapped around each individual wire of the pair. Therefore significant differences in the value of increased capacitance between wires of a pair is created by such shield, and as this difference in capacitance increases with increase in length of the wire pair, delay in the flow of signals through wires of the pair also changes, introducing significant additional intra-pair skew. In the extreme case of wire twist imbalance, where one wire is twisted around another that is more or less straight, most of the increase in capacitance is on the longer, twisted, outer wire adjacent to the shield wrap around the pair. Hence wire delay, which was significantly greater for the outer wire due to its greater length in this instance, increases even more for the outer wire due to additional capacitance to the foil shield. Shielding implemented in this manner (foil wrap), therefore, amplifies intra-pair skew due to wire length and dimensional differences in twisted wire pairs. A drain wire added to the mix also contributes to this problem since there is no definite method to ensure that the drain wire is equally coupled to both wires in a shielded twisted wire pair (STP) along the length of the STP. Hence, though addition of a foil wrap around a twisted wire pair (foil wrapped pair or FWP) and a drain wire inside this assembly contacting the foil provides a measure of shielding that minimizes coupling into or emissions from the wire pair, it adds to the original problem (intra-pair skew) creating emissions from the wire pair. More importantly, as discussed in application Ser. No. 11/654,168 and U.S. Pat. No. 7,449,639 [Ref. 3], intra-pair skew severely limits high-speed capability of wire pairs and cables over any significant length of cable, and foil wrap exacerbates this limitation. Similarly, impedance variations along the length of the FWP that existed before foil wrap, caused by dimensional or dielectric material variations, may be amplified by a foil shield around a twisted pair, degrading signal integrity further.
As the definition and quality of 2-D images and audio in multimedia transmission increases, and a migration to high definition (1080P, or 1920×1080 pixels, and 4K or 4096×3072 pixels/3-D displays, with 32 bits or higher per pixel for color, and at 60 up to 120 Hz screen refresh rates) proceeds, there is a clear need for significantly higher data rates (of as much as 48 Gbps) and correspondingly high frequencies of operation of links such those defined in the consumer electronics High Definition Multimedia Interface (HDMI), DisplayPort, and other similar links. In view of varied and significant limitations in prior art twisted wire pairs, their shielding, and cable assemblies, there is a need to improve upon wire pairs and cable architecture for such links.